Production method for light emitting element

ABSTRACT

In a first invention, a p-type Mg x Zn 1-x O-type layer is grown based on a metal organic vapor-phase epitaxy process by supplying organometallic gases which serves as a metal source, an oxygen component source gas and a p-type dopant gas into a reaction vessel. During and/or after completion of the growth of the p-type Mg x Zn 1-x O-type layer, the Mg x Zn 1-x O-type thereof is annealed in an oxygen-containing atmosphere. This is successful in forming the layer of p-type oxide in a highly reproducible and stable manner for use in light emitting device having the layer of p-type oxide of Zn and Mg. In a second invention, a semiconductor layer which composes the light emitting layer portion is grown by introducing source gases in a reaction vessel having the substrate housed therein, and by depositing a semiconductor material produced by chemical reactions of the source gas on the main surface of the substrate. A vapor-phase epitaxy process of the semiconductor layer is proceed while irradiating ultraviolet light to the main surface of the substrate and the source gases. This is successful in sharply enhancing reaction efficiency of the source gases when the semiconductor layer for composing the light emitting layer portion is formed by a vapor-phase epitaxy process, and in readily obtaining the semiconductor layer having only a less amount of crystal defects. In a third invention, a buffer layer having at least an Mg a Zn 1-a O-type oxide layer on the contact side with the light emitting layer portion is grown on the substrate, and the light emitting layer portion is grown on the buffer layer. The buffer layer is oriented so as to align the c-axis thereof to the thickness-wise direction, and is obtained by forming a metal monoatomic layer on the substrate based on the atomic layer epitaxy, and then by growing residual oxygen atom layers and the metal atom layers. This is successful in obtaining the light emitting portion with an excellent quality. In a fourth invention, a ZnO-base semiconductor active layer included in a double heterostructured, light emitting layer portion is formed using a ZnO-base semiconductor mainly composed of ZnO containing Se or Te, so as to introduce Se or Te, the elements in the same Group with oxygen, into oxygen deficiency sites in the ZnO crystal possibly produced during the formation process of the active layer, to thereby improve crystallinity of the active layer. Introduction of Se or Te shifts the emission wavelength obtainable from the active layer towards longer wavelength regions as compared with the active layer composed of ZnO having a band gap energy causative of shorter wavelength light than blue light. This is contributive to realization of blue-light emitting devices.

TECHNICAL FIELD

This invention relates to a light emitting device and a method offabricating the same.

BACKGROUND ART

There have long been demands for high-luminance, light emitting devicecapable of causing short-wavelength emission in the blue light region.Such light emitting device has recently been realized by usingAlGaInN-base materials. Rapid progress has also been made in applyingthe device to full-color, light emitting apparatuses or to displayapparatuses by combining it with red and green high-luminance, lightemitting devices. Use of the AlGaInN-base material, however, inevitablyraises the costs because the material contains Ga and In as majorcomponents, both of which are relatively rare metals. One of other majorproblems of the material is that the growth temperature thereof is ashigh as 700 to 1,000° C., and thus consumes a considerably large amountof energy for the production. This is undesirable not only in terms ofcost reduction, but also in terms of being against the stream of thetimes where discussions on energy saving and suppression of globalwarming are prevailing. Japanese Laid-Open Patent Publication No.2001-44500 proposes a light emitting device having a more inexpensiveZnO-base compound semiconductor layer heteroepitaxially grown on asapphire substrate. Japanese Laid-Open Patent Publication No. 11-168262discloses a two-dimensional-array planar light emitting device using alight emitting layer portion composed of oxides of Zn and Mg, or alloythereof.

In addition, an InAlAsP/InGaAsP compound semiconductor laser typicallyfor use in transponders for submarine optical fiber cables, of whichspecifications such as crystal defect density or the like are verystrictly regulated in order to realize a high output and an highdurability.

In all of these devices, semiconductor layers composing the lightemitting layer portion are formed by a vapor-phase epitaxy process suchas sputtering, molecular beam epitaxy (MBE) or metal organic vapor phaseepitaxy (MOVPE).

There is a problem that oxide layers of Zn and Mg are very likely tocause oxygen deficiency, and they inevitably tend to have an n-typeconductivity, so that it is intrinsically difficult to obtain thecrystal having only a less amount of n-type carrier (electrons) as aconductive carrier. Nevertheless, in the fabrication of the electronicdevices disclosed in the above-described patent publications, it isessentially necessary to form oxide layers of Zn and Mg having a p-typeconductivity. These oxide crystals, however, tend to have an n-typeconductivity due to oxygen deficiency as described in the above, and ithas long been believed as very difficult to form the p-type crystal ornon-doped, semi-insulating crystal used for the active layer. Onepossible method may be such as adding p-type dopant, but conversion ofan n-type conductivity of a material into a p-type conductivity needs alarge amount of dopants in order to compensate the whole portion of theexisting n-type carriers and to excessively generate p-type carriers, sothat problems in stability, reproducibility and uniformity of theelectric characteristics remain unsolved.

Even for the case where the light emitting device is to be fabricated bya vapor-phase epitaxy process using any compound semiconductors otherthan the oxides of Zn and Mg (referred to as ZnO-base oxide orMgZnO-base oxide, hereinafter), only a tiny crystal defect ascribable tovariation in reaction efficiency of the source gases may cause failureespecially in the aforementioned InAIAsP/InGaAsP compound semiconductorlaser, for which a very high level of quality is required, and mayconsiderably lower the production yield.

ZnO-base oxide can be obtained by a vapor-phase epitaxy in a vacuumenvironment, where heteroepitaxial growth process using a substrate of adifferent origin, such as sapphire, is unconditionally adopted becauseof difficulty in bulk single crystal growth. It is therefore necessaryto interpose an appropriate buffer layer between the substrate and thelight emitting layer portion in order to attain a desirablecrystallinity of the light emitting layer portion as described in theabove. The aforementioned Japanese Laid-Open Patent Publication No.2001-44500 discloses a method in which the buffer layer (contact layer)is formed by MBE (Molecular Beam Epitaxy) process or MOVPE (MetalorganicVapour Phase Epitaxy) process similarly to the light emitting layer tobe formed in succession.

The MBE process, however, cannot readily suppress generation of theoxygen deficiency due to its low pressure in the growth atmosphere, sothat it is very difficult for the process to form the ZnO-base oxidelayer which is indispensable for composing the light emitting device. Onthe other hand, the MOVPE process can arbitrarily vary partial pressureof oxygen during the growth, and thus can suppress generation of theoxygen elimination or oxygen deficiency by raising the atmosphericpressure to some extent. In the MOVPE process generally proceeded in acontinuous manner, even if any accidental irregularity such asdeficiency or dislocation of the atoms should occur, the layer growthfor the next layer and thereafter continuously proceed while leaving theirregularity unrepaired, so that the process could not always ensure adesirable quality of the buffer layer which governs the crystal qualityof the light emitting layer portion, and this has consequently beenmaking it difficult to obtain the device having an excellent lightemission efficiency.

The aforementioned ZnO-base oxide will have a larger band gap energy asalloy composition x of MgO (magnesium oxide) to ZnO (zinc oxide)increases. For the case where the ZnO-base semiconductor light emittingdevice, which comprises an MgZnO-type oxide, is configured based on thedouble heterostructure, it is therefore a general practice to composethe active layer with ZnO in view of ensuring more effective confinementof carriers injected thereto. The MgZnO-type oxide can be formed by theMOVPE process or MBE process as described in the above, but theformation process thereof is highly causative of oxygen deficiency ofthe MgZnO-type oxide and can readily result in degradation ofcrystallinity of the active layer composed of ZnO. This consequentlyexpands total half value width of the emission wavelength rangeascribable to the active layer, reduces the emission intensity, andsuppresses the emission efficiency for specific wavelength to bedesired.

A first subject of the invention is, therefore, to provide a method offabricating a light emitting device having a ZnO-base oxide layer,capable of growing the p-type oxide layer in a reproducible and stablemanner.

A second subject of the invention is to provide a method of fabricatinga light emitting device capable of drastically raising reactionefficiency of the source gases when the semiconductor layer composingthe light emitting layer portion is formed by a vapor-phase epitaxyprocess, and of readily realizing semiconductor layers having aconductivity type which have not conventionally been obtainable, andhaving only a less amount of crystal defects and being high in quality.

A third subject of the invention is to provide a method of fabricating alight emitting device capable of realizing a high-quality, lightemitting layer portion composed of a ZnO-base oxide, and to provide alsoa light emitting device obtainable by the method.

A fourth subject of the invention is to provide a light emitting deviceusing a ZnO-base oxide, of which active layer can be formed with a highquality in an exact manner, and is further to provide ahigh-performance, blue-color light emitting device at low costs.

DISCLOSURE OF THE INVENTION

(First Invention)

A first invention is to solve the aforementioned first subject, and isto provide a method of fabricating a light emitting device having alight emitting layer portion which includes a p-type Mg_(x)Zn_(1-x)O(where, 0≦x≦1) layer, wherein the p-type Mg_(x)Zn_(1-x)O layer is grownby a metal organic vapor-phase epitaxy process while supplyingorganometallic gases, an oxygen component source gas and a p-type dopantgas into a reaction vessel, and is annealed during and/or aftercompletion of the growth thereof in an oxygen-containing atmosphere.

In the first invention, the p-type Mg_(x)Zn_(1-x)O layer grown by ametal organic vapor-phase epitaxy process is annealed in theoxygen-containing atmosphere during and/or after completion of thegrowth. This effectively prevents the oxygen deficiency from occurring,and successfully obtains a crystal having a less amount of n-typecarrier. It is therefore no more necessary to add an excessive amount ofp-type dopant for compensating the n-type carrier, and this makes itpossible to obtain the light emitting device containing the p-typeMg_(x)Zn_(1-x)O layer, excellent in the stability, reproducibility anduniformity in the electrical characteristics.

In order to obtain a high-luminance, light emitting device, it iseffective to compose the light emitting layer portion so as to have adouble heterostructure as described in the next. That is, the lightemitting layer portion is configured so as to have a structure in whichan n-type cladding layer, an active layer, and a p-type Mg_(x)Zn_(1-x)O(where, 0≦x≦1) layer are stacked in this order. The method offabricating a light emitting device according to the first inventionherein characteristically comprises:

-   -   an n-type cladding layer growing step for growing the n-type        cladding layer; and    -   an active layer growing step for growing the active layer; and    -   a p-type cladding layer growing step for growing the p-type        cladding layer by a metal organic vapor-phase epitaxy process        while supplying organometallic gases, an oxygen component source        gas and a p-type dopant gas into a reaction vessel, and        annealing the p-type cladding layer during and/or after        completion of the growth thereof in an oxygen-containing        atmosphere. This method is successful in realizing, a device        showing high emission intensity specific to the double        heterostructure.

The light emitting layer portion can be configured so that the n-typecladding layer composed of an n-type Mg_(z)Zn_(1-z)O (where, 0≦z≦1)layer, the active layer composed of a Mg_(y)Zn_(1-y)O (where, 0≦y<1,x>y) layer, and the p-type cladding layer composed of a p-typeMg_(x)Zn_(1-x)O (where, 0≦x≦1) layer are stacked in this order. In then-type cladding layer formation step herein, organometallic gases and anoxygen component source gas are supplied into the reaction vessel so asto allow the n-type cladding layer to grow on the substrate based on ametal organic vapor-phase epitaxy process. The active layer growing stepherein is a step for growing the active layer on a substrate by a metalorganic vapor-phase epitaxy process while supplying organometallic gasesand an oxygen component source gas into the reaction vessel, andincludes a step for annealing the layer during and/or after completionof the growth thereof in an oxygen-containing atmosphere.

In the above-described method in which the active layer composed of theMg_(y)Zn_(1-y)O layer and the p-type cladding layer composed of thep-type Mg_(x)Zn_(1-x)O layer are formed by a metal organic vapor-phaseepitaxy process, the annealing carried out during and/or aftercompletion of the growth of these layers can effectively prevent theoxygen deficiency from occurring within the layers, and is successful inreadily obtaining the crystal having only a less amount of n-typecarrier. It is therefore no more necessary for the p-type cladding layerto be added with an excessive amount of p-type dopant for compensatingthe p-type carrier, and it is made possible for the active layer tosuppress the carrier concentration and to raise the emissionrecombination efficiency. This is also advantageous in that it canlargely reduce the costs, because all layers composing the lightemitting layer portion can be composed using the inexpensive MgZnO-baseoxide material. On the other hand, the growth process for the n-typecladding layer does not adopt the aforementioned annealing to therebyintentionally produce the oxygen deficiency (note that a composite oxideobtained by partially replacing Zn in ZnO with Mg is occasionallyabbreviated as MgZnO in the description below, this by no meansindicates a condition of Mg:Zn:O=1:1:1, where the same will apply alsoto the second to fourth inventions).

It is now preferable to suppress the oxygen deficiency concentration inthe p-type MgZnO layer or MgZnO active layer to as low as 10 sites/cm³or below (0 site/cm³ not precluded). In this case, it is very difficultfor RF sputtering and molecular beam epitaxy (MBE) to suppressgeneration of the oxygen deficiency, since pressure in the growthatmosphere in these processes are as low as 10⁻⁴ Torr to 10⁻² Torr(1.3332×10⁻² Pa to 1.3332 Pa), so that it is substantially impossiblefor these methods to grow the p-type MgZnO layer. On the contrary, avapor-phase epitaxy process based on the MOVPE process can arbitrarilyvary oxygen partial pressure during the growth, and thus can suppressgeneration of the oxygen elimination or oxygen deficiency by raising theatmospheric pressure to some extent.

When the annealing for suppressing generation of oxygen deficiency iscarried out, it is preferable to reduce as possible the amount of supplyof the organometallic gases than the amount of supply adopted for thecase where the layer growth is a matter of preference, and it is morepreferable to interrupt the supply, in view of suppressing generation ofthe oxygen deficiency in the layers. The oxygen-containing atmosphereduring the annealing can be created by introducing the oxygen componentsource gas (same as that used for the layer growth based on the MOVPEprocess) into the reaction vessel, which is efficient because theannealing can be completed within the same reaction vessels used for thelayer growth.

The annealing may be carried out after completion of the layer growth,but it may be difficult for the annealing after the completion to fullyremove the oxygen deficiency which remains deep inside the layer if theoxygen deficiency is accidentally formed in the process of the layergrowth. It is therefore effective to carry out the annealing during thelayer growth, and more preferably to alternatively repeat theintermittent layer growth and the annealing in the oxygen-containingatmosphere for the purpose of more effective suppression of the oxygendeficiency. In this case, the aforementioned repetition of theintermittent layer growth and annealing will be more efficient if thelayer to be annealed is grown while continuously supplying the oxygencomponent source gas and intermittently interrupting supply of theorganometallic gases, to thereby make use of the time duration ofinterrupted supply of the organometallic gases as an effective durationof the annealing.

Next, the annealing for suppressing the oxygen deficiency for the p-typeMgZnO layer or the MgZnO active layer must be carried out in theoxygen-containing atmosphere having an oxygen partial pressure higherthan dissociation oxygen pressure of MgZnO (where, oxygen-containingmolecules other than O₂ are to be included after converting thecomponent oxygen into O₂). In an atmosphere having an oxygen partialpressure lower than the dissociation oxygen pressure of MgZnO, it isimpossible to prevent the oxygen deficiency from occurring due topromoted decomposition of MgZnO. The oxygen partial pressure adaptableto the annealing is more preferably 1 Torr (133.32 Pa) or above. Whilethere is no special limitations on the upper limit of the oxygen partialpressure, the pressure is preferably set within a range not causative ofunnecessary rise in costs of the annealing facility (typically set at7,600 (1.013 MPa) Torr or around for the annealing in the reactionvessel).

(Second Invention)

A second invention is to solve the aforementioned second subject, and isto provide a method of fabricating a light emitting device having a stepof growing a semiconductor layer for composing a light emitting layerportion in vapor phase by introducing source gases in a reaction vesselhaving a substrate disposed therein, and by allowing a semiconductormaterial generated based on chemical reactions of the source gases todeposit oh the main surface of the substrate, wherein a vapor-phaseepitaxy process of the semiconductor layer is proceeded whileirradiating ultraviolet light to the source gases introduced in thereaction vessel.

Because the chemical reactions for producing the semiconductor materialfrom the source gases is promoted by ultraviolet irradiation in thesecond invention, the semiconductor material will be less causative ofcrystal defects or the like during deposition on the main surface of thesubstrate, and will readily realize the semiconductor layer having onlya less amount of crystal defects.

In the production of the semiconductor material through the chemicalreactions of the source gases, a reaction system containing the sourcegases needs be transferred into a reactive transition state having ahigh enthalpy. If the amount of energy required for causing transfer tothe transition status is not supplied, unreacted or incompletely-reactedcomponents of the source gases will increase components causingadsorption within the layer and will be causative of the crystaldefects. Although the necessary energy might be supplemented by heatenergy, this requires rise in the temperature of the system. Anexcessive rise in the temperature of the substrate however ruinadsorption ratio of the semiconductor material contributable to thecrystal growth, and undesirably results in formation of the layers onlyhaving a large amount of crystal defects. In contrast, combined use ofthe ultraviolet irradiation described in the above is successful insecuring a necessary and enough energy for completing the generationreactions of the semiconductor material without excessively raising thetemperature of the system, and in forming the semiconductor layer havingonly a less amount of crystal defects.

In this case, one possible system is such as having a ultraviolet lightsource disposed so as to oppose with the main surface of the substrate,in which the source gases are supplied between the substrate and theultraviolet light source while irradiating ultraviolet light towards themain surface. This is successful in selectively accelerating thegeneration reactions of the semiconductor material from the source gasesin the vicinity of the main surface of the substrate. Ultraviolet lightirradiated to the substrate is once absorbed by the substrate, and canhighly activate the outermost portion of the layers under growth basedon the light excitation effect. More specifically, it is supposed that ahighly activated status similar to that obtainable by the layer growthunder a high temperature is locally realized in the outermost portion ofthe layers, and this makes it possible to efficiently proceed the layergrowth while suppressing thermal decomposition of the source gascomponents in the vapor phase.

In one rational method of irradiating ultraviolet light to the sourcegases or the substrate in the reaction vessel, a part of the wallportion of the reaction vessel opposing to the main surface of thesubstrate is configured as a transparent wall portion, the ultravioletlight source is disposed outside the reaction vessel, and ultravioletlight from the ultraviolet light source is irradiated towards the mainsurface through the transparent wall portion. According to thisconfiguration, the ultraviolet light source can be disposed outside thereaction vessel, and this prevents the light source per se from beingadversely affected by corrosion or deposited reaction products, andelongates the service life of the apparatus.

Although any vapor-phase epitaxy processes may be applicable so far asthey can correlate the chemical reactions to the layer growth, a metalorganic vapor-phase epitaxy (MOVPE) process is particularly preferablebecause of its potential of efficiently growing a high-quality oxidesemiconductor or compound semiconductor. While the MBE process is onepossible method other than the MOVPE process, the MOVPE process can moreadvantageously be adopted to the formation of the oxide semiconductorlayer described below because it is more unlikely cause the oxygendeficiency.

In a metal organic vapor-phase epitaxy process, the semiconductor layercomposed of the metal oxide can be formed by using organometallic gasesand an oxygen component source gas as the source gases, based onchemical reactions of the organometallic gases with the oxygen componentsource gas. In the formation of the oxide semiconductor, any unreactedor incompletely-reacted oxygen component source gas incorporated intothe layer by adhesion will be causative of the oxygen deficiency afterelimination of the component. The oxygen deficiency emits an electron asa carrier, and thus inevitably makes the conductivity type of theresultant layer n-type. This is a serious non-conformity in formation ofp-type layer or insulating (non-doped) layer indispensable for formingthe light emitting layer portion. Adoption of the second inventionherein is successful in effectively suppressing generation of the oxygendeficiency. The oxide semiconductor layer thus formed is exemplified byMg_(x)Zn_(1-x)O (where, 0≦x≦1) layer. Use of the Mg_(x)Zn_(1-x)O layermakes it possible to readily form a light emitting device capable ofensuring high luminance light emission in the blue light region orultraviolet region.

Adoption of the second invention is successful in effectivelysuppressing the oxygen deficiency, and is consequently successful inreadily obtaining the crystal having only a less amount of n-typecarrier. It is therefore no more necessary to add an excessive amount ofp-type dopant for compensating the n-type carrier, and this makes itpossible to obtain the light emitting device containing the p-typeMg_(x)Zn_(1-x)O layer, excellent in the stability, reproducibility anduniformity in the electrical characteristics.

More specifically, the light emitting layer portion can be configured soas to have a double heterostructure in which the n-type cladding layercomposed of an n-type Mg_(z)Zn_(1-z)O (where, 0≦z≦1) layer, the activelayer composed of a Mg_(y)Zn_(1-y)O (where, 0≦y<1, x>y) layer, and thep-type cladding layer composed of a p-type Mg_(x)Zn_(1-x)O (where,0≦x≦1) layer are stacked in this order. In this case, the n-typecladding layer can readily be formed by supplying the organometallicgases and an oxygen component source gas into the reaction vessel,without specifically irradiating ultraviolet light. The active layer canbe formed by supplying the organometallic gasses and an oxygen componentsource gas into the reaction vessel with irradiating ultraviolet light.The p-type cladding layer can be formed by additionally supplying ap-type dopant gas in the process similar to that for the active layer.

Also in the second invention, it is preferable to suppress the oxygendeficiency concentration in the p-type MgZnO layer or MgZnO active layerto as low as 10 sites/cm³ or below (0 site/cm³ not precluded), and avapor-phase epitaxy process based on the MOVPE process is preferable inview of suppressing the oxygen deficiency.

The second invention is also applicable to fabrication of compoundsemiconductor light emitting devices other than those using theMgZnO-base oxide, such as InAIAsP/lnGaAsP compound semiconductor lightemitting device (laser device, in particular).

(Third Invention)

A third invention is to solve the aforementioned third subject, andincludes a method of fabricating a light emitting device andthus-fabricated, light emitting device. The method of fabricating alight emitting device of the third invention is such as fabricating alight emitting device having a light emitting layer portion composed ofan Mg_(a)Zn_(1-a)O-type (where, 0≦a≦1) oxide, wherein a buffer layer isformed on a substrate, the buffer layer having at least anMg_(a)Zn_(1-a)O-type oxide layer on the contact side with the lightemitting layer portion, and the light emitting layer portion is grown onthe buffer layer;

-   -   the Mg_(a)Zn_(1-a)O-type oxide layer has wurtzite crystal        structure in which metal atom layers and oxygen atom layers are        alternatively stacked in the direction of the c-axis, the buffer        layer is grown so as to orient the c-axis of the wurtzite        crystal structure to the thickness-wise direction, and so as to        form a metal atom layer as a metal monoatomic layer on the        substrate by the atomic layer epitaxy, and then to form the        residual oxygen atom layers and metal atom layers.

The light emitting device of the third invention is such as having alight emitting layer portion composed of an Mg_(a)Zn_(1-a)O-type (where,0≦a≦1) oxide and formed on a substrate, and having a buffer layer formedbetween the substrate and the light emitting layer portion, the bufferlayer having at least an Mg_(a)Zn_(1-a)O-type oxide layer on the contactside with the light emitting layer portion;

-   -   the Mg_(a)Zn_(1-a)O-type oxide layer has wurtzite crystal        structure in which metal atom layers and oxygen atom layers are        alternatively stacked in the direction of the c-axis; and    -   the buffer layer has the c-axis of the wurtzite crystal        structure oriented to the thickness-wise direction, has a single        atom layer portion as a metal monoatomic layer formed in contact        with the substrate, and has the residual oxygen atom layers and        metal atom layers alternatively stacked successive to the metal        monoatomic layer.

In the third invention, the entire portion or at least a portion on thecontact side with the light emitting layer portion of the buffer layerformed on the substrate is composed of an Mg_(a)Zn_(1-a)O-type oxide(where, alloy composition a is not always same with that of the lightemitting layer portion, and the oxide may occasionally be referred to asMgZnO-type oxide or simply as MgZnO, while omitting indication of thealloy composition a). Because the portion on the junction interface sideof the buffer layer and the light emitting layer portion have basicallythe same crystal structure (wurtzite crystal structure) and the samecomponent system, local irregularity of the crystal structure due tointeraction between the components over the junction interface becomesless likely to occur, and this is advantageous in realizing the lightemitting layer portion having a desirable crystallinity. Typically theentire portion of the buffer layer may be composed of the MgZnO-typeoxide. This makes it possible to carry out a vapor-phase epitaxy processof the buffer layer and light emitting layer portion in the samefacility in an extremely simple manner.

In the third invention, the buffer layer is formed particularly so as toform a metal atom layer as a metal monoatomic layer on the substrate bythe atomic layer epitaxy (ALE) process, and then to form the residualoxygen atom layers and metal atom layers. By adopting the ALE process,formation of the metal atom layer can be saturated once a single atomiclayer is completed (so-called, self-termination function), and the atomsarranged in the layer are less likely to cause any irregularity such asdeficiency or dislocation. By forming a single layer of theless-irregular metal atom layer and then forming the succeeding metalatomic layers and oxygen atom layers, it is made possible to obtain thebuffer layer having an excellent crystallinity. This consequentlyimproves the crystallinity of the light emitting layer portion formedthereon, and is advantageous in realizing a high-performance, lightemitting device. By adopting the above-described method, the lightemitting device of the third invention will have the c-axis of thewurtzite crystal structure oriented to the thickness-wise direction,will have the single atom layer portion in contact with the substrateformed as a metal monoatomic layer, and will have the residual oxygenatom layers and metal atom layers alternatively formed in succession tothe metal monoatomic layer. Thus-configured buffer layer has anexcellent crystallinity, and this makes it possible to realize the lightemitting layer portion having only a less amount of defects andirregularity, and having a desirable emission efficiency.

The ALE process can be carried out in a form of a metal organicvapor-phase epitaxy (MOVPE) process in which an organometallic compoundgas and an oxygen component source gas are supplied in a reaction vesselhaving a substrate disposed therein. More specifically, only anorganometallic compound gas, which serves as a source material for themetal atom layer, is allowed to flow through the reaction vessel tothereby form the first metal atom layer for composing the buffer layerso as to be saturated by a single atom layer, to thereby form a metalmonoatomic layer. As shown in FIG. 16A, organometallic compound (MO)molecule causes decomposition or elimination of organic groups boundthereto, and allows its metal atom to chemically adsorb onto thesubstrate. Under the ALE process, the metal atom is adsorbed whilekeeping a part of its organic groups unremoved, and as shown in FIG.16B, forms the metal atom layer so as to orient the residual organicgroup towards the upper-surface. Once the first single atomic layer iscompleted, thus-oriented organic groups can inhibit adhesion ofnewly-coming metal atoms and can fully exhibit the self-terminationfunction, so that the atoms arranged in the layer will become veryunlikely to cause irregularities such as deficiency and dislocation.

In the MOVPE process, oxygen partial pressure during the growth canarbitrarily be varied, so that generation of the oxygen elimination oroxygen deficiency is effectively avoidable by raising the atmosphericpressure to some extent. This consequently makes it possible to form thep-type Mg_(a)Zn_(1-a)O layer indispensable for the light emittingdevice, in particular the p-type Mg_(a)Zn_(1-a)O layer such as having adensity of oxygen deficiency of as small as 10 sites/cm³ or below. Thesmaller density of oxygen deficiency is the better (that is, 0 site/cm³not precluded).

When the MOVPE process is adopted, composition of the entire portion ofthe buffer layer using MgZnO-type oxide is advantageous, because thebuffer layer and light emitting layer portion can be grown sequentiallyin the same reaction vessel only by adjusting ratio of theorganometallic gasses and oxygen component source gas. This is alsoadvantageous in that the purging of the vessel between growth processesfor the buffer layer and light emitting layer portion needs only a shorttime as compared with the case where the buffer layer is formed usingdifferent materials such as GaN, or the purging per se is omissible.

Also in the third invention, it is effective to grow the light emittinglayer portion so as to have a double heterostructure as described below,in order to obtain a high-luminance, light emitting device. That is, thedouble heterostructured, light emitting layer portion is formed on thebuffer layer by sequentially stacking a first-conductivity-type claddinglayer (p-type or n-type) composed of Mg_(a)Zn_(1-a)O-type oxide, anactive layer, and a second-conductivity-type cladding layer (n-type orp-type) having a conductivity type different from that of thefirst-conductivity-type cladding layer, in this order.

(Fourth Invention)

A fourth invention is to provide a light emitting device for solving thefourth subject. The light emitting device has a double heterostructured,light emitting layer portion which comprises an active layer andcladding layers, wherein the active layer is composed of a Group Il-VIcompound semiconductor containing Zn as a Group II element, andcontaining O together with Se or Te as a Group VI element, and thecladding layers are composed of Mg_(x)Zn_(1-x)O-type (where, 0≦x≦1)oxide.

In the double hetero-type, ZnO-base semiconductor light emitting devicescomposed of an MgZnO-base oxide, those having the active layer composedof ZnO, having a band gap energy of 3.25 eV, causes light emission innear violet color. To adjust the band gap energy suitable for blue-colorlight emission, it is necessary to add some impurity to the ZnO activelayer to thereby form impurity levels, or to configure the active layerusing a ZnO-base alloyed compound semiconductor having a smaller bandgap energy than ZnO has.

To achieve blue-color light emission with high emission efficiency, itis necessary for the active layer to satisfy the above-describedconstitutional conditions, and to stabilize the crystallinity. In viewof stabilizing the crystallinity of the active layer composed of aZnO-base semiconductor mainly containing ZnO, an essential point residesin that how successfully the oxygen deficiency can be suppressed whenthe active layer is stacked by epitaxially growing the ZnO-basesemiconductor typically based on the MOVPE or MBE process.

The active layer in the fourth invention is formed using Group II-VIcompound semiconductor (aforementioned ZnO-base semiconductor)containing Zn (zinc) as a Group II element, and containing O (oxygen)together with Se (selenium) or Te (tellurium) as a Group VI element, andthis makes it possible to introduce Se or Te, which belongs to the sameGroup with oxygen, into the oxygen-deficient sites. For the case wherethe introduced Se or Te acts as an impurity, Zn—Se pair or Zn—Te pair issupposed to form a deeper impurity level than ZnO forms, so thatblue-color light emission With a higher efficiency than that given byZnO-base semiconductor can be obtained.

In the active layer composed of ZnO-base semiconductor, Se or Teintroduced into the oxygen-deficient sites may not exist in a form ofimpurity, but may form a local crystal structure of ZnOSe or ZnOTe whichis different from ZnO. Both of the ZnOSe crystal and ZnOTe crystal havesmaller band gap energies as compared with that of ZnO crystal, and canform the active layer capable of blue-color light emission at a higherefficiency. The emission possibly obtained via the impurity levelsresults in saturation of effect of improving the emission efficiency dueto a limited range of formation of Zn—Se pair or Zn—Te pair which iscausative of the impurity levels. On the other hand, the emissionpossibly obtained via the bands formed by the ZnOSe crystal or ZnOTecrystal results in further increase in the emission efficiency.

The double heterostructure adopted for the light emitting device of thefourth invention is such as having the active layer, which is composedof the aforementioned Se- or Te-containing, ZnO-base semiconductor,sandwiched between the cladding layers which are composed ofMg_(x)Zn_(1-x)O-type (0≦x≦1) oxide having a band gap energy larger thanthat of the active layer. The Mg_(x)Zn_(1-x)O-type-(0≦x≦1) oxide willhave a larger band gap energy as MgO alloy composition x increases, butwill also have a larger insulating property. Increase in MgO alloycomposition x, therefore, makes it difficult to dope an effective numberof carriers into the cladding layer. It is in particular difficult forZnO, having an n-type conductivity in a non-doped status, to form thep-type cladding layer which should be doped with p-type carriers. Incontrast to that the active layer has been formed by using ZnO, theactive layer in the fourth invention is formed by using the Se- orTe-containing, ZnO-base semiconductor having a band gap energy smallerthan that of ZnO, so that it is made possible to configure the claddinglayer using the Mg_(x)Zn_(1-x)O-base oxide of which ZnO or MgO alloycomposition x is suppressed to a low level. This consequently makes itpossible to dope an effective number of carriers into the claddinglayer, to dope an effective number of carriers also into the activelayer, and to improve the emission efficiency.

When the active layer is composed of ZnOSe crystal or ZnOTe crystal, theZnOSe crystal or ZnOTe crystal will have a smaller band gap energy asthe ratio of Se or Te to O increases, and thus the emission wavelengthbecomes shorter. A band gap energy suitable for blue-color lightemission falls within a range from 2.52 to 3.15 eV, where the largestband gap energy of 3.15 eV suitable for blue-color light emission can beattained typically by adjusting a ratio of O and Se to 61:39 for theZnOSe crystal, and by adjusting a ratio of O and Te to 81:19 for theZnOTe crystal. Because ZnO has a band gap energy of 3.25 eV, thecladding layer can be formed by using ZnO without suppressing thecarrier confinement effect in the active layer. By composing thecladding layer with ZnO, the cladding layer and active layer will haveZnO as a major constituent thereof, and this not only makes it possibleto improve working efficiency in the fabrication, but also makes itunnecessary to use excessive Mg, and contributes to cost reduction.

Beside the above-described blue-color light emission, it is alsopossible to obtain band gap energy suitable for emission at longerwavelength regions such as blue-green to green regions, by adjustingratio of Se and Te to O in the ZnOSe crystal or ZnOTe crystal. Since theband gap energy of the active layer in this case is smaller than thatsuitable for blue-color light emission, the cladding layer can becomposed by using ZnO.

The active layer of the ZnO-base semiconductor light emitting device ofthe fourth invention can be configured as having a multi-layeredstructure in which sub-layers composed of ZnSe or ZnTe are inserted in amain layer composed of ZnO so as to be distributed over thethickness-wise direction.

As described in the above, when the active layer is formed byepitaxially growing ZnO-base semiconductor, crystallinity of the activelayer can be improved by introducing Se or Te, which belongs to the sameGroup with oxygen, to oxygen-deficient sites. It is also possible toshift the emission wavelength of the active layer to the longerwavelength region. While the active layer may be configured as a singlelayer composed of Se- or Te-containing ZnO-base semiconductor, adoptionof the above-described, multi-layered structure, which is typified by astructure in which the sub-layers composed of ZnSe or ZnTe, and having awidth not larger than that of a unimolecular layer of the active layer,are inserted in a main layer composed of ZnO, ensures the effectsdescribed in the next. Thus-formed sub-layer can function as a δ dopedlayer and can localize Se or Te in the thickness-wise direction, andthis makes it possible to enhance effect of introducing Se or Te to theoxygen-deficient sites. This enhances binding tendency with Zn in theclosest vicinity, and raises probability of forming Zn—Se pair or Zn—Tepair, or of forming ZnOSe crystal or ZeOTe crystal. Even if the devicesare not introduced into the oxygen-deficient sites, it is made possibleto prevent non-luminescent center caused by unmatched interface ordislocation, by suppressing formation of different crystal phases suchas ZnSe and ZnTe. If the coverage ratio of the sub-layer is controlledso as to be smaller than a unimolecular layer of the active layer, Se orTe is successfully prevented from depositing as an impurity rather thanbeing incorporated into the oxygen-deficient sites.

Because the number of layers of the sub-layers to be inserted into theactive layer can properly be selected depending on the band gap energy,and more specifically on the ratio of Se or Te to O in the ZnOSe crystalor ZnOTe crystal for composing the active layer, and is not specificallylimited. It is, however, preferable of course that effect ofintroduction of Se or Te can uniformly extend over the active layer inview of obtaining a uniform light emission therefrom. It is thereforepreferable to form the sub-layers so as to be distributed over thethickness-wise direction, and typically in a periodical manner.

Other conditions commonly applicable to the first to fourth inventionswill be described.

The growth of the p-type MgZnO layer or MgZnO active layer based on theMOVPE process can more advantageously be proceeded under an atmosphereconditioned at a pressure of 10 Torr (1.3332 kPa) or above, so as tomore effectively suppress generation of the oxygen deficiency during thefilm formation, and to obtain the p-type MgZnO layer or MgZnO activelayer having desirable characteristics. It is more preferable herein toadjust oxygen partial pressure (including any other oxygen-containingmolecules other than O₂, after converting component oxygen to O₂) to 10Torr (1.3332 kPa) or above. For the case where the n-type MgZnO layer isformed on the buffer layer, and further thereon the MgZnO active layerand p-type MgZnO layer is formed, any oxygen deficiencies generated inthe n-type MgZnO may be causative of irregularity or the like in theMgZnO active layer and p-type MgZnO layer formed thereafter, so that itis preferable that also the n-type MgZnO layer is grown so as tosuppress the oxygen deficiency as possible. In this case, the n-typeMgZnO layer is added with an n-type dopant so as to have theconductivity type of n-type. On the other hand, for the case where thep-type MgZnO layer is formed on the buffer layer, and further thereonthe MgZnO active layer and n-type MgZnO layer are formed, it is alsoallowable to intentionally form the oxygen deficiency in the n-typeMgZnO layer so as to have an n-type conductivity.

To make Mg_(a)Zn_(1-a)O to have a p-type conductivity, it is necessaryto add an appropriate p-type dopant as described in the above. As thep-type dopant, either one of, or two or more of N, Ga, Al, In, Li, Si,C, and Se are available. Among these, use of N is particularlypreferable in view of obtaining desirable p-type characteristics. As themetal element dopant, either one of, or two or more of Ga, Al, In and Liare available, where Ga is particularly effective. Combined addition ofthese dopants with N can ensure desirable p-type characteristics in amore reliable manner.

To ensure sufficient emission characteristics, p-type carrierconcentration in the p-type Mg_(a)Zn_(1-a)O layer preferably fallswithin a range from 1×10¹⁶ sites/cm³ to 8×10¹³ sites/cm³. The p-typecarrier concentration less than 1×10¹⁶ sites/cm³ may make it difficultto obtain a sufficient emission luminance. On the other hand, the p-typecarrier concentration exceeding 8×10¹⁸ sites/cm³ may excessivelyincrease the amount of p-type carriers injected to the active layer, andthis is causative of increase in p-type carrier not contributable to thelight emission due to reverse diffusion into the p-type Mg_(a)Zn_(1-a)Olayer, or injection into the n-type cladding layer after getting overthe potential barrier, to thereby lower the emission efficiency. Alsofor the n-type Mg_(a)Zn_(1-a)O layer, it is preferable to adjust n-typecarrier concentration within a range from 1×10¹⁶ sites/cm³ to 8×10¹⁸sites/cm³ based on the same reason.

Examples of materials available for substrate include aluminum oxide,gallium oxide, magnesium oxide, gallium nitride, aluminum nitride,silicon, silicon carbide, gallium arsenide, indium-tin composite oxideand glass. Particularly preferable forms of the substrate include thefollowings. As shown in FIG. 2, Mg_(a)Zn_(1-a)O-type oxide has wurtzitecrystal structure comprising metal atom layers and oxygen atom layersalternatively stacked in the direction of c-axis, where the oxygen atomsfollow a hexagonal atomic arrangement. The substrate is, therefore,preferably an oxide single crystal substrate in which oxygen atomsfollow the hexagonal atomic arrangement, and the C-plane ((0001) plane)of the hexagonal atomic arrangement is exposed to the main surface, interms of improving crystal matching with the buffer layer, and ofobtaining the light emitting layer portion with a desirablecrystallinity. In this case, the buffer layer is composed of theMg_(a)Zn_(1-a)O-type oxide over the entire portion thereof, and isformed on the main surface of the oxide single crystal substrate so asto orient the c-axis of its wurtzite crystal structure in thethickness-wise direction. Examples of such oxide single crystalsubstrate include those composed of corundum-structured oxide, where asapphire substrate is one specific example thereof.

As shown in FIG. 15, in an oxide having corundum-type structure, alattice of oxygen (O) atoms has a hexagonal atomic arrangement, and inthe direction of c-axis thereof, O atom (ion) layers and metal atom(ion: shown as Al in the drawing) layers are alternatively stacked. Inthis crystal structure, one of both atomic layers appearing on both endsin the direction of c-axis will always be an oxygen atom layer plane,and the other will always be a metal atom layer plane. The O atom layerplane has the same O atomic arrangement with the O atom layer in thewurtzite crystal structure except for difference in the latticeconstants. For the case where the main surface of the oxide singlecrystal substrate having such crystal structure will have formed thereonthe buffer layer comprising Mg_(a)Zn_(1-a)O-type oxide having thewurtzite crystal structure, a junction structure having better matchingproperty can be obtained by stacking the metal atom layer of the bufferlayer on the main surface of the substrate composing the O atom layerplane.

It is to be noted that it is also allowable to grow the light emittinglayer portion on the A-plane of the sapphire substrate as disclosed inJapanese Laid-Open Patent Publication No. 2001-44500, and this iseffective to a certain extent in terms of planarization of the crystalgrowth surface. Because the A-plane of the sapphire substrate has metalatoms and oxygen atoms exposed thereon in a mixed manner, the generalcontinuous-growth-type MOVPE process may raise probability of causingadsorption of oxygen atoms and zinc atoms at the same time on theA-plane ((11-20) plane). This is more likely to cause irregularity inthe stacking of the buffer layer grown based on the c-axis orientation,and is not always successful in obtaining a high-quality buffer layerand light emitting layer portion. Use of the ALE process, as in thethird invention, is now successful in obtaining a high-quality bufferlayer and, consequently, the light emitting layer portion in a highlyreproducible manner, because the metal monoatomic layer can be formedalso on the A-plane in a forced manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual drawing of a double heterostructured, lightemitting layer portion including a p-type MgZnO layer;

FIG. 2 is a schematic drawing of a crystal structure of MgZnO;

FIG. 3 is a schematic drawing of an arrangement of the metal ions andoxygen ions in the MgZnO layer;

FIG. 4 is a schematic band chart of a light emitting device using ajunction structure of Type-I band lineup;

FIG. 5A is schematic drawings for explaining a growth process of thelight emitting layer portion of the light emitting device having a typeshown in FIG. 4 in Embodiment 1 of the invention;

FIG. 5B is a schematic sectional view of a reaction vessel shown in FIG.5A;

FIG. 6 is a drawing for explaining an exemplary fabrication process ofthe light emitting device having a type shown in FIG. 4;

FIG. 7A is a drawing for explaining operation of the method offabricating the light emitting device of the Embodiment 1 of theinvention;

FIG. 7B is an explanatory drawing as continued from FIG. 7A;

FIG. 7C is an explanatory drawing as continued from FIG. 7B;

FIG. 8A is a drawing of a first example of a supply sequence for anorganometallic gas and an oxygen component source gas in the processshown in FIG. 5A;

FIG. 8B is a drawing of a second example of the same;

FIG. 8C is a drawing of a third example of the same;

FIG. 8D is a drawing of a fourth example of the same;

FIG. 9A is a drawing of a fifth example of the same;

FIG. 9B is a drawing of a sixth example of the same;

FIG. 10A is a schematic band chart of a light emitting device using ajunction structure of Type-I and Type-II band lineups;

FIG. 10B is a schematic band chart of another example;

FIG. 11A is a schematic drawing for explaining a vapor phase growthapparatus for growing the light emitting layer portion using aultraviolet lamp based on the MOVPE process in Embodiment 2 of theinvention;

FIG. 11B is a schematic drawing of a modified example of FIG. 11A;

FIG. 12 is a conceptual drawing of a vapor-phase epitaxy process forforming the light emitting layer portion using ultraviolet laser beam;

FIG. 13 is a schematic drawing showing a specific example of the lightemitting device of Embodiment 3 of the invention;

FIG. 14A is a drawing for explaining an exemplary fabrication process ofthe light emitting device shown in FIG. 13;

FIG. 14B is a drawing of a process step as continued from FIG. 14A;

FIG. 14C is a drawing of a process step as continued from FIG. 14B;

FIG. 15 is a schematic drawing of corundum-type crystal structure;

FIG. 16A is a drawing for explaining operation of a method offabricating a light emitting device of Embodiment 3 of the invention;

FIG. 16B is a drawing for explaining operation as continued from FIG.16A;

FIG. 16C is a drawing for explaining operation as continued from FIG.16B;

FIG. 16D is a drawing for explaining operation as continued from FIG.16C;

FIG. 17 is a drawing exemplifying a temperature control sequence and gassupply sequence in the process steps shown in FIGS. 14A to 14C;

FIG. 18A is a drawing for explaining effect of configuring a mixed metalatom layer as a metal monoatomic layer grown by the ALE process;

FIG. 18B is a drawing for explaining the effect as continued from FIG.18A;

FIG. 18C is a drawing for explaining the effect as continued from FIG.18B;

FIG. 19A is a drawing for explaining an example in which a metalcomposition gradient layer is configured as a buffer layer;

FIG. 19B is a drawing for explaining the metal composition gradientlayer shown in FIG. 19A;

FIG. 20A is a schematic sectional view showing a stacked structure of afirst example of the ZnO-base semiconductor light emitting device ofEmbodiment 4 of the invention;

FIG. 20B is a schematic sectional view showing a stacked structure of asecond example of the same;

FIG. 21 is a schematic sectional view showing a stacked structure of anexemplary electrode formation status of the ZnO-base semiconductor lightemitting device of the fourth embodiment of the invention; and

FIG. 22 is a schematic sectional view showing a stacked structure ofanother exemplary electrode formation status differed from FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Best modes for carrying out the invention will be explained referring tothe drawings.

Embodiment 1

FIG. 1 is a drawing schematically showing a stacked structure of theessential portion of the light emitting device of the first invention,and the device has a light emitting layer portion in which an n-typecladding layer 34, an active layer 33 and a p-type cladding layer 2 arestacked in this order. The p-type cladding layer 2 is composed as ap-type Mg_(x)Zn_(1-x)O layer (0≦x≦1: may occasionally be referred to asp-type MgZnO layer 2, hereinafter). In the p-type MgZnO layer 2, a traceamount of either one of, or two or more of N, Ga, Al, In and Li, forexample, are contained as a p-type dopant. The p-typecarrier-concentration is adjusted within a range from 1×10¹⁶sites/cm³ to8×10¹⁸ sites/cm³ as described in the above, and more specifically withina range from 10¹⁷ sites/cm³ to 10¹⁸ sites/cm³ or around.

FIG. 2 is a schematic drawing of a crystal structure of MgZnO, whereso-called wurtzite structure is shown. In this structure,oxygen-ion-packed planes and metal-ion (Zn ion or Mg ion) packed planesare stacked along the direction of the c-axis alternatively, and asshown in FIG. 3, the p-type MgZnO layer 2 is formed so as to align thec-axis thereof along the thickness-wise direction. Formation of avacancy due to omission of an oxygen ion causes oxygen deficiency, andconsequently produces electrons as n-type carriers. Excessive formationof such oxygen deficiency undesirably increases the n-type carriers, tothereby ruin p-type conductivity. It is therefore important that howcompletely the oxygen deficiency can be suppressed in order to form thep-type MgZnO layer.

The p-type MgZnO layer 2 can be formed by the MOVPE process. Principleof the MOVPE process per se is publicly known. The aforementioned p-typedopant is added during a vapor-phase epitaxy process. The p-type MgZnOlayer 2 is annealed during or after completion of a vapor-phase epitaxyprocess in an oxygen-containing atmosphere. The annealing is successfulin suppressing elimination of oxygen ions, and in obtaining a desirablep-type MgZnO layer 2 having only a small amount of oxygen deficiency. Itis also effective to carry out the growth of the p-type MgZnO layer 2under an atmospheric pressure of 10 Torr (1.3332 kPa) or above in termsof suppressing generation of the oxygen deficiency.

Now referring back to FIG. 1, the active layer 33 is composed of amaterial having an appropriate band gap depending on desired emissionwavelength. For example, for those available for visible light emission,materials having band gap energies E_(g) (3.10 eV to 2.18 eV or around),capable of causing light emission in a wavelength range of 400 nm to 570nm, are selected. Although this range covers emission wavelength fromviolet region to green region, those having band gap energies E_(g)(2.76 eV to 2.48 eV or around) capable of causing light emission in awavelength range of 450 to 500 nm are selected in particular for thecase where blue-color light emission is desired. On the other hand,those having band gap energies E_(g) (4.43 eV to 3.10 eV or around)capable of causing light emission in a wavelength range of 280 nm to 400nm are selected in particular for the case where ultraviolet emission isdesired.

The active layer 33 can be formed typically using a semiconductorcapable of forming a Type-I band lineup between itself and the p-typeMgZnO layer. An example of such active layer 33 is an Mg_(y)Zn_(1-y)Olayer (where, 0≦y<1, x>y: referred to as MgZnO active layer,hereinafter). It is to be noted now that “a Type-I band lineup is formedbetween the active layer and the p-type MgZnO layer” means a junctionstructure, as shown in FIG. 4, in which the individual energy levels ofthe bottom of the conduction band and the upper end of the valence bandE_(cp), E_(vp) of the p-type cladding layer (p-type MgZnO layer 2), andthe individual energy levels of the bottom of the conduction band andthe upper end of the valance band E_(ci), E_(vi) of the active layersatisfy the following relations of inequality:E_(ci)<E_(cp)  (1)E_(vi)>E_(vp)  (2)

In this structure, specific barrier will appear for both of the forwarddiffusion of holes from the active layer 33 to the n-type cladding layer34, and the forward diffusion of electrons (n-type carriers) to thep-type cladding layer 2. If the material for the n-type cladding layer34 is appropriately selected so as to form Type-I band lineup betweenthe active layer 33 and the n-type cladding layer 34, similarly to asshown in FIG. 4, the active layer will have formed therein well-formedpotential barriers both at the bottom of the conduction band and theupper end of the valence band, and will enhance confinement effect bothfor electrons and holes. This consequently results in more enhancedeffects of promoting carrier recombination and of improving emissionefficiency. While AlGaN or the like is available for the n-type claddinglayer 34, n-type Mg_(z)Zn_(1-z)O layer (where 0≦z≦1:occasionally alsoreferred to as n-type MgZnO layer”, hereinafter) is more advantageous,because this makes it possible to form all layers composing the lightemitting layer portion with MgZnO-base oxide material (such lightemitting layer portion will be referred to as “full-oxide-type, lightemitting layer portion, hereinafter), so that it is no more necessary touse rare metals such as above-described Ga and In (dopants excluded),which contributes to a considerable cost reduction. Height of thepotential barriers on both sides of the active layer can be equalized bymaking the alloy compositions of the n-type MgZnO layer 34 and thep-type MgZnO identical. Thickness t of the active layer 33 is selectedso as to avoid decrease in the carrier density in the active layer 33and excessive increase in the amount of carriers passing through theactive layer 33 based on the tunneling effect, and is typically adjustedwithin a range from 30 nm to 1,000 nm.

In the MgZnO active layer 33, a value of alloy composition y can, alsoserve as a factor which determines band gap energy E_(g). For example,the value is selected in a range of 0≦y≦0.5 for the case whereultraviolet emission over a wavelength of 280 nm to 400 nm is desired.The potential barrier height thus formed is preferably 0.1 eV to 0.3 eVor around for light emitting diode, and 0.25 eV to 0.5 eV or around forsemiconductor laser light source. This value can be determined byselecting the alloy compositions x, y and z of the p-typeMg_(x)Zn_(1-x)O layer 2, Mg_(y)Zn_(1-y)O active layer 33, and n-typeMg_(z)Zn_(1-z)O layer 34.

The following paragraphs will describe one exemplary process forfabricating the light emitting device having the aforementionedfull-oxide-type, light emitting layer portion. First, as shown in (a) ofFIG. 6, a GaN buffer layer 11 is epitaxially grown on a sapphiresubstrate 10, and a p-type MgZnO layer 52 (typically of 50 nm thick), anMgZnO active layer 53 (typically of 30 nm thick), and an n-type MgZnOlayer 54 (typically of 50 nm thick) are formed in this order (invertedorder of the growth also acceptable). The epitaxial growth of theindividual layers can be carried out by the MOVPE process as describedin the above. It is to be noted that, MBE in the context of thisspecification include not only MBE in a narrow sense in which both of ametal element component source and a non-metal element component sourceare used in solid forms, but also include MOMBE (Metal Organic MolecularBeam Epitaxy) using the metal element component source in a form oforganometallic compound and the non-metal element component source in asolid form; gas source MBE using the metal element component source in asolid form and the non-metal element component in a gas form; andchemical beam epitaxy (CBE) using the metal element component source ina form of organometallic compound and the non-metal element componentsource in a gas form.

All of the p-type MgZnO layer 52, MgZnO active layer 53 and the n-typeMgZnO layer 54 can continuously be formed by the MOVPE process using thesame source materials and in the same reaction vessel as shown in FIG.5A. In this case, the growth is preferably allowed to proceed atslightly lower temperatures, typically at 300° C. to 400° C., so as toreduce reactivity with the GaN buffer layer (not shown in FIG. 5A), andto raise the lattice matching property. The substrate can be heatedusing a heater embedded in a susceptor for holding the substrate, asshown in FIG. 5B.

Examples of the major materials for composing the individual layers aresuch as follows:

-   -   oxygen component source gas: preferably supplied in a form of        oxidative compound gas in view of suppressing an excessive        reaction with organometallic compounds described later, although        oxygen gas is allowable, typified by N₂O, NO, NO₂ and CO, where        N₂O (nitrous oxide) adopted in this embodiment;    -   Zn source (metal component source) gas: dimethyl zinc (DMZn),        diethyl zinc (DEZn), etc.; and    -   Mg source (metal component source) gas: bis-cyclopentadienyl        magnesium (Cp₂Mg), etc.

Examples of the p-type dopant gas include the followings:

-   -   Li source gas: n-butyl lithium, etc.;    -   Si source gas: silicon hydrides such as monosilane;    -   C source gas: hydrocarbons (typically alkyl containing one or        more C atoms); and    -   Se source gas: hydrogen selenide, etc.

One or more selected from the group consisting of Al, Ga and In can beallowed to function as excellent p-type dopants when added together withN. Examples of the dopant gas include the followings:

-   -   Al source gas: trimethyl aluminum (TMAI), triethyl aluminum        (TEAI), etc.;    -   Ga source gas: trimethyl gallium (TMGa), triethyl gallium        (TEGa), etc.; and    -   In source gas: trimethyl indium (TMIn), triethyl indium (TEln),        etc.

For the case where N is used as a p-type dopant together with a metalelement (Ga), the p-type MgZnO layer is grown while supplying a gaswhich serves as an N source together with an organometallic gas whichserves as a Ga source. In particular in this embodiment, N₂O used as anoxygen component source also serves as an N source.

The individual source gases are fed into the reaction vessel after beingappropriately diluted with a carrier gas (nitrogen gas, for example).Ratio of flow rates of the organometallic compound gases MO whichrespectively serves as Mg source and Zn source is controlled using massflow controllers MFC or the like, corresponding to variety in the alloycomposition of the individual layers. Also flow rates of N₂O, which isan oxygen component source gas, and a p-type dopant source gas arecontrolled by the mass flow controllers MFC.

The n-type MgZnO layer 54 can be grown by a method in which oxygendeficiency is intentionally produced so as to attain n-typeconductivity, where it is effective to lower the atmospheric pressure(lower than 10 Torr (1.3332 kPa), for example) than that in the caseswhere the MgZnO active layer 53 and the p-type MgZnO layer 52 is formed.It is also allowable to form the layer by separately introducing ann-type dopant. It is still also allowable to increase ratio of Group IIto Group VI elements (supply II/VI ratio) of the source materials.

For the growth of the MgZnO active layer 53 and p-type MgZnO layer 52, aunique method capable of suppressing oxygen deficiency as described inthe next is adopted. That is, as expressed by two patterns (a) and (b)shown in FIG. 5A, the layer is grown while continuously supplying anoxygen component source gas (N₂O), whereas intermittently interruptingsupply of the organometallic gases, to thereby make use of the timeduration of interrupted supply of the organometallic gases as aneffective duration of the annealing for suppressing generation of theoxygen deficiency, or for repairing the undesirably generated oxygendeficiency.

The oxygen deficiency is caused by elimination of oxygen during thelayer growth. To suppress the oxygen deficiency, it is thereforeessential to fully react metal ions (Zn and Mg) derived from theorganometallic gases with oxygen derived from the oxygen componentsource gas. Because bond energy between oxygen and Zn or Mg isrelatively large, oxygen once bound with the metals in a stoichiometricmanner will become less likely to be eliminated again. It is, however,considered that oxygen tends to be eliminated in an intermediate statewhere the reaction is not fully completed, and that the layer growth atthe lower temperature region as described in the above is particularlycausative of the oxygen deficiency due to the incomplete reaction.

It is therefore preferable, as shown in FIG. 7A, to proceed the layergrowth only to an extremely small thickness so as to prevent the oxygendeficiency from being incorporated deep inside the layer, and then, asshown in FIG. 7B, to anneal the layer while interrupting the supply ofthe organometallic gases but continuing only the supply of the oxygencomponent source gas (N₂O), because the reaction between unreactedportions of the oxygen component source gas and organometallic metalgases is promoted, and the formation of the oxygen deficiency issuppressed. Even if the oxygen deficiency should generate, it isexpected that the oxygen component source gas decomposes and generatedoxygen is adsorbed so as to repair the oxygen deficiency. Aftercompletion of the annealing over a duration of time necessary andsufficient for fully expressing these effects, the supply of theorganometallic compound gas is restarted as shown in FIG. 7C, to therebyfurther continue the layer growth. These processes are repeatedthereafter. FIG. 8A shows an exemplary supply sequence of theorganometallic gases (MO) and the oxygen component source gas. Growth ofthe MgZnO active layer 53 and the p-type MgZnO layer 52 can be proceededbasically in a similar manner, except that the dopant gas is notsupplied for the former, but supplied only for the latter.

In this case, it is necessary that the surface of the layers during theannealing is kept at a temperature higher by 100° C. or more than thelayer growth temperature and lower than the melting point of the oxide(700° C. in this embodiment), in order to promote decomposition of theoxygen component source gas, rearrangement of the adsorbed oxygen forrepairing the oxygen deficiency, and binding reaction with metal ionsalready incorporated within the layer. The temperature higher by lessthan 100° C. than the layer growth temperature may result in only aninsufficient effect of suppressing the oxygen deficiency. On the otherhand, it is self-evident that the temperature exceeding the meltingpoint of the oxide is nonsense. Because the annealing temperature is sethigher than the substrate temperature in the layer growth, it isconvenient to use a separate heater specialized for the annealing,besides a heater for heating the substrate. The separate heater isexemplified by an infrared lamp in FIG. 5A.

Once the oxygen deficiency is formed in the newly-grown portion of thelayer, it is advantageous to anneal the layer before the oxygendeficiency is buried in view of smoothly repairing it under milderconditions. It is therefore effective to set a unit of the discontinuous(intermittent) layer growth to monoatomic layer (adjacent oxygen packinglayer and metal ion packing layer are deemed to comprise monoatomiclayer) or around. Introduction period s for the organometallic compoundgas is thus set so as to afford an amount of introduction of the gasnecessary for the growth of the monoatomic layer.

The introduction of the organometallic compound gases may be effected ina period s′ longer than the period s for forming a complete monoatomiclayer as shown in FIG. 9A, or may be effected in a shorter period s” asshown in FIG. 9B, so far as it falls within a range from 0.5 atomiclayers to 2 atomic layers. The introduction period s less than0.5-atomic-layers-equivalent time may lower the fabrication efficiency,and the exceeding 2-atomic-layers-equivalent time may reduce the meritof the intermittent layer growth, because time of annealing forsuppressing the oxygen deficiency becomes too long. The introductiontime s of the organometallic compound gases is, therefore, preferablyset considering the time required for reaction of oxygen atoms with themetal atoms, and relaxation of strain in the crystal lattice.

On the other hand, the annealing time needs some consideration. Thereaction per se between the metal atom and oxygen atom completes withina relatively short time, but an additional time is substantiallynecessary for purging of the organometallic gas out from the reactionvessel in order to ensure uniform reaction (while actual variationpattern of flow rate should always show transient periods in which flowrate of the organometallic gas varies with time, when switched from theannealing period including the purge-out time, the transient periods arenot illustrated in FIGS. 8A to 8C, and FIGS. 9A and 9B for simplicity).Assuming now that the sectional area of the reaction vessel allowing thegas flow as 20 cm² as shown in FIG. 5B, a total gas volume as 50liters/min (converted value for the standard state), and a length of theheated portion including the substrate along the gas flow direction as5.0 cm, a minimum necessary time for the purging is calculated as 0.002seconds. However, the time for purging of 0.002 seconds is practicallyinsufficient, because it is technically difficult to keep a signalinput/output cycle of a gas sequencer precisely as short as less than0.1 seconds, and a stagnation layer is formed in the vicinity of theinner wall of the reaction vessel and at the heated portion includingthe substrate, where the flow rate is slower. It is therefore preferableto set an interruption time for the introduction of the organometalliccompound as long as 1 second or more so as to tolerate the mechanicalaccuracy. Specific conditions for the annealing typically relate to anitrogen flow rate of 10 liters/min (converted value for the standardstate), N₂O flow of 1 liter/min (converted value for the standardstate), a layer surface temperature of 700° C., a pressure of 760 Torr(101.3 kPa), and a retention period for one cycle of 5 to 15 seconds.

It is also allowable to keep supply of a small amount of theorganometallic compound gas during annealing period, as shown in FIG.8B, rather than completely interrupting the supply, so far as thesuppressive effect for the oxygen deficiency will not largely be ruined.It is also allowable to reduce the supply volume of the oxygen componentsource gas from the supply volume during the layer growth as shown inFIG. 8C, because oxygen during the annealing period is necessary only inan amount consumed for suppressing or repairing the oxygen deficiency.It is still also allowable to gradually increase or decrease the amountof supply of the organometallic compound gas as shown in FIG. 8D,instead of the step-wise variation shown in FIG. 8A.

During the layer growth while introducing the organometallic gases, itis effective to keep pressure in the reaction vessel at 10 Torr (1.3332kPa) or above. This is more successful in suppressing the oxygenelimination, and in growing the MgZnO layer having a less amount ofoxygen deficiency. In particular for the case where N₂O is used as theoxygen component source, the above-described setting of the pressuresuccessfully prevents N₂O from being rapidly dissociated, and this makesit possible to more effectively suppress generation of the oxygendeficiency. The higher the atmospheric pressure rises, the larger asuppressive effect for the oxygen elimination becomes, where a pressureof only as high as 760 Torr (1 atm, or 101.3 kPa) or around may besufficient for obtaining the effect. Adoption of a pressure of 760 Torr(101.3 kPa) or below means that the reaction vessel is conditioned atnormal pressure or reduced pressure, and this requires only a relativelysimple seal structure of the vessel. On the contrary, adoption of apressure exceeding 760 Torr (101.3 kPa) means that the vessel ispressurized, and this requires a slightly stronger seal structure inorder to prevent leakage of the internal gases, and further requires apressure-proof structure or the like for the case where the pressure isconsiderably high, where the suppressive effect for the oxygenelimination becomes more distinctive in anyway. The upper limit of thepressure in this case should be determined to an appropriate valueconsidering a balance between the cost of the apparatus and attainablesuppressive effect for the oxygen elimination (typically 7,600 Torr (10atm, or 1.013 MPa) or around).

After completion of the growth of the light emitting layer portion, ametal reflective layer 22 is formed on the n-type MgZnO layer 54 asshown in (b) of FIG. 6, the sapphire substrate 10 is separated as shownin (c) of FIG. 6, and a transparent conductive material layer 25 (e.g.,ITO film) is formed on the p-type MgZnO layer 52. Thereafter as shown in(d) of FIG. 6, the light emitting device 104 is obtained by dicing. Itis also allowable herein to leave the growth substrate such as sapphiresubstrate unseparated, and to use it as a part of the device.

The annealing for suppressing the oxygen deficiency for the MgZnO activelayer 53 and p-type MgZnO layer 52 may collectively be carried out afterthe layer growth completed. In this case, it is also allowable to carryout the annealing after the substrate is transferred to a separatefurnace specialized for annealing different from the reaction vessel.The annealing is preferably carried out each time the MgZnO active layer53 and p-type MgZnO layer 52 are grown. In view of repairing the oxygendeficiency incorporated into the layer, the annealing is preferablycarried out at a temperature range slightly higher than that in the casewhere the layer growth and annealing are repeated in an intermittentmanner. Specific conditions for the annealing typically relates tonitrogen flow rate of 10 liters/min (converted value for the standardstate), N₂O flow rate of 1 liter/min (converted value for the standardstate), a layer surface temperature of 800° C., a pressure of 760 Torr(101.3 kPa), and an annealing period of 30 minutes.

The active layer 33 shown in FIG. 1 can also be formed using asemiconductor capable of forming a Type-II band lineup between itselfand the p-type MgZnO layer 2. An example of such active layer 33 is anInGaN layer (referred to as InGaN active layer, hereinafter). It is tobe noted now that “a Type-II band lineup is formed between the activelayer and the p-type Mg_(x)Zn_(1-x)O layer” means a junction structureas shown in FIG. 10A, in which the individual energy levels of thebottom of the conduction band and the upper end of the valence bandE_(cp), E_(vp) of the p-type cladding layer (p-type Mg_(x)Zn_(1-x)Olayer 2), and the individual energy levels of the bottom of theconduction band and the upper end of the valance band E_(ci), E_(vi) ofthe active layer satisfy the following relations of inequality:E_(ci)>E_(cp)  (3)E_(vi)>E_(vp)  (4)

In this structure, no specific barrier will appear for the forwarddiffusion of electrons (n-type carriers) from the active layer to thep-type cladding layer, but a relatively high potential barrier is formedfor the reverse diffusion of holes (p-type carriers) from the activelayer to the p-type cladding layer. This promotes carrier recombinationin the active layer, and can achieve high emission efficiency. Assumingnow that the layer is expressed as In_(α)Ga_(1-α)N, where α is an InNalloy composition, a relation of 0.34≦α≦0.47 is preferably adopted forblue visible light emission, and a relation of 0≦α≦0.19 is preferablyadopted for ultraviolet emission.

In this case, the n-type cladding layer 34 preferably uses asemiconductor capable of forming a Type-I band lineup between itself andthe active layer. An example of such n-type cladding layer 34 is ann-type AlGaN (Al_(β)Ga_(1-β)N) layer. It is to be noted now that “aType-I band lineup is formed between the n-type cladding layer and theactive layer” means a junction structure, as shown in FIG. 10A, in whichthe individual energy levels of the bottom of the conduction band andthe upper end of the valence band E_(ci), E_(vi) of the active layer,and the individual energy levels of the bottom of the conduction bandand the upper end of the valance band E_(ci), E_(vi) of the n-typecladding layer (n-type AlGaN layer 4) satisfy the following relations ofinequality:E_(ci)<E_(cn)  (5)E_(vi)>E_(vn)  (6)

In this structure, a relatively high potential barrier is formed for thereverse diffusion of electrons from the n-type cladding layer to theactive layer, and a well-type potential barrier is formed at the upperend of the valence band corresponding to the position of the activelayer, to thereby enhance the confinement effect of holes. All of thesepromote carrier recombination in the active layer, and consequentlyachieve high emission efficiency.

In the structures shown in FIG. 10A, a suppressive effect of reversediffusion of holes from the active layer to the p-type cladding layercan successfully be raised by increasing the energy barrier height(E_(vi)−E_(vp)) at the upper end of the valence band. For this purpose,it is effective to raise MgO alloy composition of the p-typeMg_(x)Zn_(1-x)O layer 2 (that is, value of x) composing the p-typecladding layer. The alloy composition x is determined depending ondesired current density, so as not to cause excessive leakage of thecarriers towards the p-type cladding layer. In a typical case where theactive layer 33 is composed of an InGaN layer, the alloy composition xis preferably set within a range from 0.05 to 0.2 or around for lightemitting diode, and 0.1 to 0.4 or around for semiconductor laser lightsource.

The bottom of the conductive band descends in a step-wise manner fromthe active layer towards the p-type cladding layer, and the electronsnot contributed to the emissive recombination in the active layer thenflow into the p-type cladding layer having a higher carrierconcentration, and become no more contributable to light emission due toAuger recombination or the like. In order to raise the emissionefficiency, it is therefore necessary that electrons as much as possiblerecombine with holes before they flow into the p-type cladding layer,and it is therefore effective to increase the thickness t of the activelayer to a certain level or above (e.g., 30 nm or above). As shown inFIG. 10B, too small thickness t of the active layer increases electronspossibly flow into the p-type cladding layer and become notcontributable to the light emission, and this results in loweredemission efficiency. On the other hand, increase in the thickness t ofthe active layer beyond a necessary level results in lowered carrierdensity in the active layer and thus lowers the emission efficiency. Thethickness is thus typically set to 2 μm or below.

In FIG. 10A, it is advantageous in view of suppressing non-emissiverecombination at the junction boundary that a relation of E_(cp)>E_(vi)is satisfied similarly for the case where the InGaN active layer isused, that is, the p-type cladding layer and the active layer haveforbidden bands which overlap with each other.

Embodiment 2

The next paragraphs will describe an embodiment of the second invention.Since the essential portion of the light emitting device to which thesecond invention is applicable is same as described in Embodiment 1,detailed description will be omitted (see FIGS. 1 to 4, and FIGS. 10Aand 10B). As shown in (a) of FIG. 6, the GaN buffer layer 11 isepitaxially grown again on the sapphire substrate 10, and furtherthereon the p-type MgZnO layer 52 (typically of 50 nm thick), the MgZnOactive layer 53 (typically of 30 nm thick) and the n-type MgZnO layer 54(typically of 50 nm thick) are formed in this order (inverted order ofthe growth also acceptable). The epitaxial growth of the individuallayers in this embodiment can be carried out by the MOVPE processsimilarly to as described in Embodiment 1, where differences reside inthe following points. More specifically, in the growth of the MgZnOactive layer 53 and p-type MgZnO layer 52 herein, a ultraviolet lamp(e.g., excimer ultraviolet lamp) as a ultraviolet light source isdisposed opposing to the main surface of the substrate in order tosuppress the generation of the oxygen deficiency, and the source gasesare supplied between the substrate and ultraviolet light source whileirradiating ultraviolet light from the ultraviolet lamp towards the mainsurface of the substrate.

FIGS. 11A and 11B show an apparatus used for vapor-phase epitaxy processof the light emitting layer portion using the ultraviolet lamp based onthe MOVPE process. Similarly to as described previously referring toFIG. 5A, all of the p-type MgZnO layer, MgZnO active layer and n-typeMgZnO layer can sequentially be formed in the same reaction vessel usingthe same source gases. In this case, it is preferable to proceed thegrowth at slightly lower temperatures, typically at 300 to 400° C. so asto reduce the reactivity with the GaN buffer layer and raise the latticematching property. The substrate can be heated using a heater embeddedin a susceptor for holding the substrate.

The wall portion of the reaction vessel is configured as a transparentwall portion composed of a quartz glass or the like, and the ultravioletlamp is disposed outside the reaction vessel, so as to effect theultraviolet irradiation through the transparent wall portion towards thesubstrate. The ultraviolet lamp available herein has an emissionwavelength of approximately 172 nm, and an output power density ofapproximately 8 mW/cm² when the flow rates of N₂O and organometalliccompound gas are within a range from 100 cm³/min to 1,000 cm³/min and 10cm³/min to 100 cm³/min, respectively.

It is supposed that ultraviolet light irradiated to the substrate isonce absorbed by the substrate, and can highly activate the outermostportion of the layers under growth based on the light excitation effect.That is, a highly activated status similarly to as obtained in the layergrowth under high temperatures can locally be realized in the outermostportion of the layer. Also a part of the source gases is brought into ahigh-energy transition status (radical, etc.) by the ultravioletirradiation. As a consequence, the organometallic gases and oxygencomponent source gas (N₂O) can react in the vicinity of the activatedoutermost portion of the layer, in a stoichiometric manner withoutcausing unreacted components or the like, and the layer growth ispromoted in a manner less causative of the oxygen deficiency.

Radicals of the organometallic gases and oxygen component source gas areunstable in general, and the radicals ascribable to these componentswill be converted into other decomposition products not contributable tothe oxide formation reaction, if a status in which these radicals arebrought into a close vicinity enough for causing reaction is notrealized for a long duration of time. While this kind of decompositionreaction is more likely to proceed as temperature of the systemelevates, this can be suppressed to a certain extent typically bylowering the substrate temperature to as relatively low as 400° C. orbelow. The ultraviolet irradiation can enhance reaction activity in thevicinity of the main surface of the substrate, and this makes itpossible to readily form the oxide semiconductor layer having only aless amount of oxygen deficiency even when the substrate temperaturecannot be raised so high for various reasons.

On the other hand, probability of the oxide formation reactioncontributable to the layer growth is higher in the boundary layer (inwhich mass transfer is governed by diffusion, also referred to asstagnation layer), and lower in an area outside the boundary layer andhaving a large gas flow rate. It is thus understood that the larger theflow rate of the gas flowing through the reaction vessel grows, thethinner the boundary layer becomes, and the growth speed of the oxide isdepressed. Adjusting now the flow rate of the source gases suppliedbetween the substrate and the ultraviolet lamp (ultraviolet lightsource) so as to be faster on the ultraviolet light source side thanthat on the main surface side as shown in FIG. 11B, the reactionproducts become less likely to deposit on the wall portion of thereaction vessel in the vicinity of the ultraviolet lamp, and this makesit possible to avoid a nonconformity such that the deposit shadowsultraviolet light from the light source to thereby degrade the reactionefficiency. More specifically, as shown in FIG. 11B, a gas intake portand a gas discharge port of the reaction vessel are formed so as to bedivided into a first gas intake/discharge port, and a second gasintake/discharge port, and the flow rate is adjusted so as to make a gasflow rate λ₁ on the first gas intake/discharge port side is faster thana gas flow rate λ₂ on the second gas intake/discharge port side.

The ultraviolet lamp is advantageous in view of ensuring a largeirradiation area, and of allowing the reaction for the oxide layerformation to proceed in a uniform and efficient manner. On the otherhand, it is also allowable to irradiate a ultraviolet laser beam in atwo-dimensional scanning manner over the substrate as shown in FIG. 12.This system can use a light convergence density larger than thatavailable from the ultraviolet lamp, and thus further enhance thereaction efficiency. In an exemplary configuration shown in FIG. 12, alaser light source-composed as an excimer laser light source or asemiconductor laser light source is scanned in the X direction with theaid of a polygon mirror, and in synchronization therewith a susceptorholding the substrate is driven in the Y direction, which crosses the Xdirection, with the aid of a Y-scanning table, so as to scan over theentire portion of the main surface of the substrate with the laser beamin a two-dimensional manner.

The process steps after completion of the growth of the light emittinglayer portion are same as those described in Embodiment 1 referring to(b) to (d) of FIG. 6.

Embodiment 3

The next paragraphs will describe an embodiment of the third invention.Although the essential portion of the light emitting device to which thethird invention is applicable is almost the same as described inEmbodiment 1 (see FIGS. 1 to 5A, and FIGS. 10A and 10B), it is essentialin the third invention to form the buffer layer as described below. Thatis, the buffer layer has the c-axis of the wurtzite crystal structureoriented to the thickness-wise direction, has a single metal atom layeras a metal monoatomic layer formed in contact with the substrate, andhas the residual oxygen atom layers and metal atom layers alternativelystacked successive to the metal monoatomic layer. An exemplaryfabrication process will be explained below.

First as shown in FIG. 13, a buffer layer 111 composed of MgZnO isepitaxially grown on the sapphire substrate 10, and further thereon ann-type MgZnO layer 34 (typically of 50 nm thick), an MgZnO active layer33 (typically of 30 nm thick) and a p-type MgZnO layer 32 (typically of50 nm thick) are formed in this order (inverted order of the growth orlayers 32 to 35 also acceptable). These layers can be grown by the MOVPEprocess.

By the MOVPE process, all of the buffer layer 111, n-type MgZnO layer34, MgZnO active layer 33 and p-type MgZnO layer 32 can continuously beformed by the MOVPE process using the same source materials and in thesame reaction vessel as shown in FIGS. 14A to 14C. Temperature in thereaction vessel is adjusted using a heating source (an infrared lamp inthis embodiment) so as to promote the chemical reactions for the layergrowth. Major source materials for the individual layers and style offeeding thereof are the same as those described in Embodiments 1 and 2.

The buffer layer 111 is grown as descried in the next. FIG. 17 shows acontrol sequence of temperature in the reaction vessel and introductionof the individual gases in this embodiment. The substrate 10 on whichthe layers are grown is a sapphire (i.e., single crystal alumina)substrate having the c-axis as the principal crystal axis, where themain surface on the oxygen-exposed plane side shown in FIG. 15 is usedas a layer growth plane. Prior to the layer growth, the substrate 10 isthoroughly annealed under an oxidative gas atmosphere. The oxidative gasmay be any of those selected from O, CO and N₂O, where N₂O is selectedin this embodiment so as to be used also as the oxygen component sourcegas in the layer growth described later. For the case where theannealing is carried out in the reaction vessel for the MOVPE process,preferable conditions for the annealing relate to a temperature of 750°C. or above (but lower than the melting point of the substrate), and anannealing time of 30 minutes or more. It is, however, also allowable toshorten the above-described annealing time if the surface of thesubstrate can be cleaned to a satisfactory level by wet cleaning or thelike.

After completion of the annealing, the substrate temperature is loweredto a first temperature which is set to 250 to 300° C. (set to 350° C.herein) as shown in FIG. 17 in order to suppress generation of theoxygen deficiency, while keeping the oxidative gas atmosphere. After thetemperature is stabilized at a set value, supply of the oxidative gas isinterrupted, and the gas is then thoroughly purged out by replacing theinner atmosphere of the reaction vessel with nitrogen gas. It ispreferable to set the purging time to 5 seconds or longer, althoughvariable depending on shape and capacity of the reaction vessel.

Next, as shown in FIGS. 14A and 16A, the organometallic gas MO issupplied into the reaction vessel, and the first metal atomic layerwhich composes a part of the buffer layer 111 is formed as a monoatomiclayer by the ALE process. As previously explained in the above, growthof the monoatomic layer in the ALE process saturates once a singleatomic layer is completed based on the self-termination function, and nomore growth of the metal atomic layer would occur even if the supply ofthe organometallic compound gas MO is continued.

Thereafter the supply of the organometallic gas MO is interrupted, thegas is thoroughly purged out by replacing the inner atmosphere of thereaction vessel with nitrogen gas, and as shown in FIG. 16C, N₂O isintroduced as the oxygen component source gas (and also as a gas forcreating the oxidative atmosphere), and the oxygen atom layer is formedonly by a single atomic layer by the ALE process. This results in theformation of the MgZnO layer only by a single atomic layer on thesubstrate 10.

The temperature in the reaction vessel is thereafter increased to asecond temperature which is set to 400 to 800° C. (set to 750° C.herein) as shown in FIG. 17 while keeping the oxidative gas atmosphere,and also keeping the organometallic gas continuously supplied, so as toform the residual portion of the buffer layer by the general MOVPEprocess as shown in FIGS. 14B and 16D. In this process, the buffer layer111 having an excellent planarity can be obtained by growing the layerat a speed of 0.1 nm/sec or around until a thickness of 10 nm or aroundis attained, and thereafter at a speed of 1 nm/sec. In view of obtainingthe buffer layer excellent both in the crystallinity and planarity, itis also preferable to grow a plurality of layers from the first layersby the ALE process.

Although the buffer layer 111 of this embodiment is formed as a simpleoxide layer comprising ZnO, it may also be formed as a composite oxidelayer of MgZnO having an appropriate alloy composition harmonized withthe alloy composition of the adjacent layer on the lightemitting-portion side. The Al atom layer located just below theoutermost oxygen atom layer of the sapphire substrate comprises, asshown in FIG. 18A, two Al atom sites Al-1 and Al-2, which differ fromeach other in the distance to the oxygen layer. Assuming now that themetal atom layer formed on the oxygen layer is a Zn atom layer, bothsites Al-1 and Al-2 differ in the Coulomb repulsive force between Znatom and Al atom located while placing the oxygen layer in between. Forthis reason, Zn atoms corresponding to both sites will have differentdisplacement in the direction normal to the plane of the oxygen atomlayer, and this may causative of irregularity in stacking of thelater-coming layers. To relieve this effect, as shown in FIGS. 18B and18C, it is effective to form the first single atomic layer (or aplurality of layers) as a composite oxide layer which contains Group IIatom (e.g., Mg) having a smaller ionic radius than Zn, or Group II atom(e.g., Ca, Sr, Ba) having a larger ionic radius by an appropriate ratio,and this can improve the crystallinity of the light emitting layerportion to be obtained. It is now also effective, in view of enhancingthe above-described effect, to dispose a composition-gradient layer,having metal cation composition gradated in the thickness-wisedirection, between such composite oxide layer (having a metal cationcomposition A) and the cladding layer (having a metal cation compositionB: n-type MgZnO layer 54 herein) formed in contact with the buffer layer111, in order to ensure continuity between both compositions A and B, asshown in FIG. 19A. In an exemplary case where both of the compositeoxide layer and cladding layer are composed of MgZnO, thecomposition-gradient layer can be formed so that composition parameter νvaries continuously between A and B typically as shown in FIG. 19B,where composition parameter ν represents metal cation composition and isgiven by ν≡N_(Mg)/(N_(Mg)+N_(Zn)), where N_(Mg) is molar content of Mg,and N_(Zn) is molar content of Zn; A is an expression of ν for thecomposite oxide layer, and B is that for the cladding layer.

After the buffer layer 111 is completed, as shown in FIG. 14C, then-type MgZnO layer 34, MgZnO active layer 33 and p-type MgZnO layer 32are formed in this order by the MOVPE process. These process steps arebasically same as those described in Embodiments 1 and 2.

In this embodiment, after completion of the growth of the light emittinglayer portion, the active layer 33 and the p-type MgZnO layer 32 arepartially removed by photolithography or the like as shown in FIG. 13, atransparent electrode 125 comprising indium tin oxide (ITO) or the likeis formed, a metal electrode 122 is formed on the residual p-type MgZnOlayer 32, and the layers are then diced together with the substrate 10to thereby produce the light emitting device 1. It is thus self-evidentthat the light emitting device 1 is configured so that the buffer layer111 composed of MgZnO is formed on the substrate 10, and further thereonthe light emitting layer portion again composed of MgZnO is formed.Light extraction is therefore available mainly on the transparentsapphire substrate 10 side.

It is to be noted that the light emitting device can of course beconfigured as shown in FIG. 6. In this case, the layers are formed onthe buffer layer 111 in an order inverted from that shown in FIG. 13,that is, the p-type MgZnO layer 32, MgZnO active layer 33 and n-typeMgZnO layer 34 are formed in this order. This configuration isadvantageous in obtaining the device having an improved weatherability,because a metal layer of MgZnO composing the light emitting layerportion is exposed only after the substrate 10 is separated.

Embodiment 4

FIGS. 20A and 20B schematically show a stacked structure of theessential portion of the light emitting device in order to explain oneembodiment of the fourth invention. As shown in FIG. 20A, on a substrate210, a ZnO buffer layer 211, an n-type MgZnO-type oxide layer 234, aZnO-base semiconductor active layer 233 and a p-type MgZnO-type oxidelayer 232 are stacked by the epitaxial growth process while keepinglattice matching, to thereby form a double hetero, light emitting layerportion 200. The ZnO-base semiconductor active layer (also simplyreferred to as active layer) 233 is composed of a ZnO-base semiconductorcontaining Zn as a Group II element, and containing O together with Seor Te as a Group VI element. FIG. 20A shows the active layer 233configured as a single layer, whereas FIG. 20B shows the active layer233 having a multi-layered structure in which sub-layers 237 composed ofZnSe or ZnTe are periodically inserted in a ZnO main layer 236 whilekeeping an area width equivalent to or less than one molecular layer ofthe active layer 233.

As shown in FIG. 20A, by composing the active layer 233 using a ZnO-basesemiconductor containing Se or Te, it is made possible to introduce Seor Te, which belongs to the same Group with oxygen, to oxygen-deficientsites, and this is successful in improving the crystallinity of theactive layer 233 and making the band gap energy thereof well suited toblue-color light emission as described in the above. On the other hand,as shown in FIG. 20B, by composing the active layer 233 so as to have amulti-layered structure in which sub-layers 237 composed of ZnSe or ZnTeare periodically inserted in a ZnO main layer 236, it is made possibleto enhance binding property of thus introduced Se or Te with the closestZn. Although FIG. 20B illustrates the sub-layer 237 as having a coverageratio of 1, it is also allowable to reduce the coverage ratio to assmaller than 1 in order to prevent Se or Te from being deposited ratherthan being introduced into the oxygen-deficient sites. The number offormation of the sub-layers 237 can properly be adjusted depending ondesired emission wavelength in the active layer 233.

The substrate 210 shown in the FIGS. 20A and 20B may be such as thoseused in Embodiments 1 to 3. Although the ZnO buffer layer 211 canepitaxially be formed by stacking ZnO crystal, it is also allowable toepitaxially grow either one of ZnS, ZnSe and ZnTe, and then convert themto obtain the ZnO buffer layer 211 by annealing under theoxygen-containing atmosphere.

N-type dopant added to the n-type, MgZnO-type oxide layer 234 (alsosimply referred to as n-type MgZnO layer 234, hereinafter) and p-typedopant added to the p-type, MgZnO-type oxide layer 232 (also simplyreferred to as p-type MgZnO layer 232, hereinafter) may be such as thoseused in Embodiments 1 to 3.

The epitaxial growth of the individual layers shown in FIG. 20A can becarried out based on the MOVPE or MBE process. It is to be noted thatMBE in the context of this patent specification include not only MBE ina narrow sense in which both of a metal element component source and anon-metal element component source are used in solid forms, but alsoinclude MOMBE (Metal Organic Molecular Beam Epitaxy) using the metalelement component source in a form of organometallic compound and thenon-metal element component source in a solid form; gas source MBE usingthe metal element component source in a solid form and the non-metalelement component in a gas form; and chemical beam epitaxy (CBE) usingthe metal element component source in a form of organometallic compoundand the non-metal element component source in a gas form.

Also the ZnO main layer 236 shown in FIG. 20B can be formed by theepitaxial growth process similarly to as described in the above. On theother hand, the sub-layer 237, which is composed of ZnSe or ZnTe, andmust be adjusted to have an area width equivalent to or less than asingle molecular layer of the active layer 233, can be formed by the ALE(Atomic Layer Epitaxy) process in which a Zn-source gas and S- orSe-source gas, both serve major source materials, are alternativelysupplied. A proper adjustment of flow rates of thus supplied sourcegases makes it possible to reduce the coverage ratio of the sub-layer237 smaller than 1.

Major source materials for the individual layers, except the Se sourceand Te source, may be such as those used in the MOVPE process inEmbodiments 1 to 3, and also the basic process steps are the same asthose described in Embodiments 1 to 3. Available Se-source gases includeH₂Se, and available Te-source gases include H₂Te.

After completion of the growth of the light emitting layer portion 200,the substrate 210 is lapped and etched, as shown in FIG. 21, a p-typeelectrode 223 composed of In and n-type electrodes 224 composed of Auare respectively formed, the stack is diced, and the individualelectrodes are bonded with Al wirings, so as to obtain the ZnO-basesemiconductor light emitting device. Light extraction is thereforeavailable mainly on the p-type MgZnO layer 232 side. In FIG. 21, thelight extraction is, however, not available from the area where thep-type electrode 223 is formed. It is therefore advantageous topartially remove the active layer 233 and p-type MgZnO layer 232 byphotolithography or the like as shown in FIG. 22, a transparentelectrode 225 comprising indium tin oxide (ITO) or the like is formed, ametal electrode 222 is formed on the residual p-type MgZnO layer 232,and the layers are then diced together with the sapphire substrate 221to thereby produce the ZnO-base semiconductor light emitting device.Light extraction is therefore available mainly on the transparentsapphire substrate 221 side.

1-19. (canceled)
 20. A light emitting device having a light emittinglayer portion composed of an Mg_(a)Zn_(1-a)O-type (where, 0≦a≦1)-oxideand formed on a substrate, and having a buffer layer formed between thesubstrate and the light emitting layer portion, the buffer layer havingat least an Mg_(a)Zn_(1-a)O-type oxide layer on the contact side withthe light emitting layer portion; the Mg_(a)Zn_(1-a)O-type oxide layerhas wurtzite crystal structure in which metal atom layers and oxygenatom layers are alternatively stacked in the direction of the c-axis;and the buffer layer has the c-axis of the wurtzite crystal structureoriented to the thickness-wise direction, has one metal atom layer as ametal monoatomic layer formed in contact with the substrate, and has theresidual oxygen atom layers and the metal atom layers alternativelystacked successive to the metal monoatomic layer.
 21. A light emittingdevice having a double heterostructured light emitting layer portionwhich comprises an active layer and cladding layers, wherein the activelayer is composed of a Group II-VI compound semiconductor containing Znas a Group II element, and containing O together with Se or Te as aGroup VI element, and the cladding layers are composed ofMg_(x)Zn_(1-x)O-type (where, 0≦x≦1) oxide.
 22. The light emitting deviceas claimed in claim 21, wherein the active layer has a multi-layeredstructure in which sub-layers composed of ZnSe or ZnTe are inserted in amain layer composed of ZnO so as to be distributed over thethickness-wise direction.
 23. The light emitting device as claimed inclaim 22, wherein the sub-layer has a width not larger than that of aunimolecular layer of the active layer.
 24. The light emitting device asclaimed in claim 21, wherein the cladding layers are composed of ZnO.